摘要:

AbstractA rigorous construction of “cross” constellations is given and the exact formulas for calculating the average norm of rectangular signal sets is derived. Using these formulas, the increase of the average norm when doubling the size of the constellation is evalua

ISSN:1124-318X    

DOI:10.1002/ett.4460080602

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

摘要:

AbstractA comparative study of arbitrarily programmable, but fixed‐rate videophone codecs using quarter common intermediate format (QCIF) video sequences scanned at 10 frames/s is offered. In contrast to existing and forthcoming standard schemes, such as the H.261, H.263 and MPEG2, MPEG4 codecs, which rely on bandwidth‐efficient but error‐sensitive variable‐length coding techniques combined with a complex self‐descriptive bitstream structure, the proposed codecs exhibit a more robust, regular bitstream and a constant bitrate. Clearly, their philosophy is different from the above error‐sensitive and variable‐rate standard schemes, since these constant‐rate codecs were designed to allow direct replacement of mobile radio voice codecs in second generation wireless systems, such as the Pan‐European GSM, the American IS‐54 and IS‐95 as well as the Japanese systems, operating at 13, 8, 9.6 and 6.7 kbit/s, respectively. This philosophy can, however, be adopted to higher‐rate systems, such as the Digital European Cordless Telecommunications (DECT) and the Universal Mobile Telecommunications System (UMTS). The Type 1 codecs proposed benefit from invoking sophisticated compression techniques in order to achieve best video quality at a given bitrate. In contrast, the Type 2 schemes introduced maximise the codecs' error resilience at the cost of slightly reduced video quality under error‐free conditions. Gain‐cost quantised, fixed but arbitrarily programmable rate discrete cosine transformed (DCT) video codecs, vector‐quantised (VQ) and quad‐tree (QT) coded algorithms are proposed and their video quality, complexity, compression ratio and error resilience trade‐offs are comparatively analysed under identical conditions. Finally, our candidate codecs are compared to the standard H261,

ISSN:1124-318X    

DOI:10.1002/ett.4460080603

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

摘要:

AbstractFive stability quantities have been considered in the new telecommunications international standards for clock specification: the Allan Deviation, the Modified Allan Deviation, the Time Deviation, the root mean square of the Time Interval Error and the Maximum Time Interval Error. A key issue to consider carefully ‐ but sometimes overlooked ‐ when performing stability measurements, in order to draw meaningful conclusions from the measurement data, is the impact of the TE sampling period on the behaviour of the stability quantities. Such a dependence, for some of the quantities above, has been already pointed out in different reference papers, which studied in depth the behaviour of the stability quantities analytically and under various aspects. In order to provide further, sourly information on the impact that the TE sampling period may have in practice on the measured stability quantities, this paper reports a broad set of simulation results, under the presence of all the power‐law clock noises and for all the stability quantities, together with some experimental results. The results obtained confirm, in excellent agreement with the theoretical analyses, that the behaviour of the Modified Allan Deviation and of the Time Deviation is substantially dependent on the chosen measurement sampling period in the observation intervals where white and flicker phase noises dom

ISSN:1124-318X    

DOI:10.1002/ett.4460080604

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

摘要:

AbstractThe measured wideband complex impulse response profiles from eight different indoor areas are used to evaluate the performance of joint diversity and equalization techniques in the presence of flat fading and frequency selective fading. The error probabilities of a joint two‐branch diversity receiver with an adaptive finite‐length equalizer on each branch, whose taps are jointly optimized according to the minimum mean square error criterion, are computed for different data rates using the measured channel respon

ISSN:1124-318X    

DOI:10.1002/ett.4460080605

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

摘要:

AbstractThe Multi Carrier Modulation (MCM) system with soft decision erasures correcting decoding based on a block code applied for Nakagami fading channel is considered. Upper and lower estimations of the Bit Error Rate (BER) have been found. Good agreement between simulated and theoretical results has been obtained. It is shown that for low SNR the proposed system has lowerBERthan MCM with Trellis Code Modulation (TCM). Moreover the decision delay needed for the proposed system is much lower than for the TCM based system.

ISSN:1124-318X    

DOI:10.1002/ett.4460080606

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

摘要:

AbstractWe design optical fiber code‐division multiple‐access (CDMA) networks to support future airborne high‐speed communication services (e.g., real‐time command and control communications). The proposed airborne communication networks have a full tunability and a fast reconfiguration time. This can support real‐time sensor data distribution and computer interconnects. Architectures for fastly tunable optical CDMA encoders and correlators using prime codes are presented, and their simple delay control logic is described, which can be easily performed by using optical fiber delay lines. Moreover, multi‐wavelength optical CDMA networks using a multi‐star topology are designed. It is shown that the use of multi‐wavelength OCDMA not only can reduce the power loss of encoders/correlators and the bandwidth expansion of optical CDMA, but also can separate avionics subsystems into groups according to different traffic requirements and data rates. This in turn can efficiently integrate several networks with different data rates and number of users into a single‐fiber network by using wavelength‐division multiplexing scheme, and new avionics subsystems can be feasibly added to such a single‐fiber network by assigning, for example, new wavelengths or additional optical CDMA codewords to facilitate future

ISSN:1124-318X    

DOI:10.1002/ett.4460080607

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

摘要:

AbstractDynamic Channel Allocation (DCA) techniques permit a high resource utilization in cellular networks and are able to adapt themselves in the presence of rapid variations of traffic loads offered to the cells. Therefore, they are particularly suitable for Mobile Satellite Systems (MSSs). This paper compares the performance of several DCA solutions that are based on the evaluation of a cost function in terms of both quality of service parameters (i.e., blocking probabilities) and signaling load to be supported by the system. Both GEostationary Orbit (GEO) and Low Earth Orbit (LEO) MSSs have been considered. A particularly interesting DCA solution is proposed that tries to serve a new call attempt in a cell where no channel is available by means of a channel reconfiguration in an interfering cell. Handover requests that do not attain immediately service can be queued for a maximum time in order to enhance system performance.

ISSN:1124-318X    

DOI:10.1002/ett.4460080608

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

摘要:

AbstractThis paper deals with the noise analysis of a special type of differential quadrature phase shift keying (QPSK) demodulator. The general principle of operation of this demodulator is discussed in a previous publication. The underlying assumptions are as follows: 1) the noise at the output of the phase‐locked loop (PLL), which is part of the demodulator, can be regarded as Gaussian‐distributed, 2) the four possible phase transitions of a QPSK‐modulated signal are equally likely and 3) the signal processing circuit following the demodulator allows perfect synchronization by means of the output symbol streams. The theoretical approach makes use of a generalized Taylor series representation that yields the output signal of a noise‐corrupted PLL. The known probabilities of error for the individual PLL output impulses allows the demodulator's symbol error rates to be determined. In order to verify the theoretical results, a hardware implementation intended for the reception with an intermediate frequency of 479.5 MHz was realized. The measurements performed on this setup are then compared with

ISSN:1124-318X    

DOI:10.1002/ett.4460080609

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY

ISSN:1124-318X    

DOI:10.1002/ett.4460080601

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1997

数据来源: WILEY